Insulating the roof against moisture – all about vapour-permeable foils

The initial roofing foil, i.e. vapour-permeable foil can be used under almost all roofing materials. It is an element ensuring roof insulation: it protects the structure, insulates the roof against wind and precipitation, as well as permeates the water vapour, which allows the roof to breathe. Find out how to choose the best roof insulation.

Initial roofing foils are vapour-permeable foils, i.e. those installed on the side of the roofing (on rafters) and allowing the water vapour to pass outside, but blocking the penetration of the dampness from outside. They protect the roof against wind blowing and getting wet. They are waterproof, which makes them an indispensable component of the complete roof insulation.

Moisture does not enter

Depending on the level of water vapour permeability, we distinguish between highly and poorly vapour-permeable roof foils. Both of them may function as the roofing for some time, that is why they are called initial roofing foils (most foils lose their elasticity under the influence of the UV radiation after two weeks since they were installed on the rafter framing; in case of foils modified with stabilizing additives, that time is prolonged to three-four or even six months). The technical data on labels inform about the kind of the foil, that is the vapour-permeability (the number of grams of the water penetrating through 1 m2 of the foil in the period of 24 hours) or inversely proportional to it Sd coefficient. The initial roofing foil should have a low Sd coefficient (less than 1 m), but higher vapour-permeability.

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The initial roofing foil can be used under almost all roofing materials. It is a roofing element which performs the insulation function – it protects the structure and roof thermal insulation against rain and snow blowing under the roofing, it prevents water and dampness from penetrating the roof. It does, however, allow water vapour to pass through, which allows the roof to breathe – and the extent to which this is possible depends on its type. It does not stiffen the roof and does not carry the weight of the roofing material – that is what the grid is for. Foils differ in the degree of water vapour permeability. It is important when designing the roof structure because the arrangement of its layers depends on the type of the foil.

Roof foils have to let the vapour pass through

Vapour-permeable initial roofing foils are usually of subdued colours – beige, grey, green, black, maroon – or they have a special anti-glare layer which prevents troublesome reflections on a sunny day and makes the work of contractors easier. It is worth to pay attention to their surface – smooth films reflect sun rays more strongly, but they are less exposed to dust settling, and water flows down them more easily. Vapour-permeable foils are thicker, rougher and more rigid than vapour-barrier foils.

Sometimes pre-wall foils are designed for use on full boarding. They always have a fibrous structure and high vapour-permeability – 1850-3000 g/m2/24h. Low-permeability foils or polyethylene foils with microporous film must not be installed on the boarding.

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Highly vapour-permeable foils

Highly vapour-permeable foils are those with the Sd coefficient within the range of 0.015-0.045 m. The amount of water vapour permeable is then above 1500-4000 g/m2/24h. They resemble paper or fabric in appearance. They have a non-woven structure, thanks to which they let the water vapour through well. Through such a foil, the water vapour can easily escape to the outside and does not accumulate in the insulation layer. As it does not impede the flow of the water vapour, it can be installed directly on the thermal insulation. Highly vapour-permeable initial roofing foils are also often called roof membranes.

Low-permeable foils

They have permeability from 20 to 100 g/m2/24h and Sd coefficient bigger than 0.1 m, so they let in the water vapour ten times less than highly vapour-permeable membranes. The roof foil cannot be in contact with the thermal insulation as it retains the water vapour which could make the insulation damp. That is why it is necessary to leave a ventilation gap of several centimetres between the foil and the thermal insulation, which will carry away the excessive dampness. In short, we can say that the vapour-permeable foil replaces traditional boarding with the roofing paper and requires a similar solution of the roof structure. It works well in unutilised attics, but it is better to use highly vapour-permeable membranes over residential rooms. Therefore, if you plan to adapt the attic after some time, it is worth using the highly vapour-permeable foil at the very beginning.

Low-permeable foils have a characteristic layered structure – they are always reinforced with a plastic net (made of polyethylene or polypropylene).

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The vapour-permeability of the foil is expressed in two ways. The first determines the amount of the water vapour in grams which passes through the surface of one square metre per day [g/m2/24 h]. The higher the value, the more vapour-permeable the foil is. Such a method of measurement is not fully authoritative as it takes into account a certain temperature and humidity. That is why some manufacturers give two values for different ambient conditions. A much simpler way to determine the vapour permeability is to give a value of the Sd [m] coefficient. It defines the diffusion resistance of the material, i.e. it compares the resistance of the water vapour penetrating the foil to the resistance of the air layer of a specified thickness. The lower the Sd coefficient value, the bigger the vapour-permeability of the foil and the higher the Sd coefficient, the better the foil blocks the water vapour.

How to install initial roofing foils

The initial roofing foils are of two colours or they have one side (the one facing up) covered with inscriptions. The purpose of that is to avoid mistakes during installation. It is important because most of the foils let the water vapour through only in one direction. Although diffusion in the polypropylene films works both ways, it is better to lay them properly because their unwoven fabric is thicker from the bottom, which enables gradual moisture release, whereas the external polyurethane coating is smoother and facilitates water drainage.

Do not start work on the roof insulation until the impregnating agent protecting the rafter framing has dried. The boards and rafters should be sanded, because they can easily damage the foil when being moved. It is safe only for the foil with smooth bottom and high strength. The installation begins with stretching the lower strip of the foil along the eaves. It is fixed to the rafters with roofing staples or wide-headed nails. The foil is slightly stretched but it should not be overtightened – it is recommended to leave a slight overhang between rafters so that the water will flow down through the created gutters and not along rafters where gaps may appear in the places of staples. Subsequent stripes of the foil are laid overlapping – in case of roofs with the inclination of more than 20°, the width of overlaps should not be smaller than 10-15 cm, and in flatter roofs – 20 cm.

The vapour-permeable foil like a sticker?

Some foils have adhesive strips and special prints making it easier to measure overlaps. Often on the edges of the foil there is also a 5-mm strip without unwoven fabric, which prevents the dampness from penetrating into overlap joints. When cutting the foil, the grid structure of the surface is also useful. Counter-battens are installed on the stretched foil, and battens are attached to them. Special sealing tape is laid under the counter-battens in order to prevent leakages in the places of fixing. The elements reaching above the roof, for example chimneys, are secured by creation of an additional gutter above them from the foil nailed to the battens. Around thin elements, such as aerials, it is enough to cut the foil crosswise and tape it tightly. All overlaps and joints should be taped to prevent leaks. It is important that those tapes are intended for a specific kind of foil, otherwise due to uneven elasticity the working foil will wrinkle, and in extreme cases it will crack. When laying the vapour-permeable foil and the roofing, one has to pay attention to maintaining all the required ventilation gaps – both on the side of the initial roofing foil and the vapour-barrier one.

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